Process for producing roll of microporous plastic film

ABSTRACT

A microporous plastic film roll production method, comprising the steps of: conveying microporous plastic film having through-holes in its interior by using a plurality of conveyance rollers at least one conveyance roller of which has a surface roughness RzJIS (μm) of 0.3≦RzJIS≦30 and has a surface made of fluorine resin, silicone rubber, or a composite material containing one of them; and winding it up in a roll.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase application of PCT/JP2012/057311, filed Mar. 22, 2012, and claims priority to Japanese Patent Application No. 2011-074642, filed Mar. 30, 2011, the disclosures of both applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing a microporous plastic film roll.

BACKGROUND OF THE INVENTION

For conventional processes in which a microporous plastic film for separators of secondary batteries and the like is conveyed and wound up into a roll, it has been very difficult to prevent wrinkle and breakage attributable to micropores.

Non-patent document 1 theoretically determines the critical wrinkling values of general films and proposes a prevention method under the title “Prevention of wrinkle”. According to this reference, the critical wrinkling value is represented in terms of tension and alignment angle, and in particular, the critical wrinkling value relating to tension depends on the thickness, Young's modulus, width, and friction coefficient of the film. As a prevention method, it proposes to adjust the tension of a film while predicting the critical slip value that is in a trade-off against wrinkle, in such a manner that it straddles the critical wrinkling value determined by the aforementioned parameters. According to knowledge of the present inventors, however, it is difficult to freely adjust the tension of a film that is low in resistance to breakage and liable to deformation in the thickness direction, such as microporous plastic film. Accordingly, it has been difficult for the aforementioned method alone to convey a film without causing either wrinkle or breakage.

Compared to this, patent document 1 proposes to add particles to the film surface and control its roughness and friction coefficient in order to solve handling-related problems, such as blobbing and wrinkle, with the polyester film used as high density magnetic recording medium. According to knowledge of the present inventors, however, the surface smoothness of microporous plastic film is not a factor in the increase in the friction coefficient, and therefore, it is difficult to prevent wrinkle by adding particles by the method proposed in patent document 1. Furthermore, particles cannot serve at all for solving the problem of breakage.

Non-patent document 2 mentions the coefficient of static friction and characteristics of substances. It is described that the coefficient of static friction of a substance is proportional to the ratio between its shear strength τ attributable to intermolecular force and its hardness H, and the friction can be reduced by using a material having a large H value and a small T value (such as silver, fluorine resin, and lead). However, the objective of this reference is to clarify the mechanism of friction and also clarify the substantial friction phenomenon under air lubrication, and it shows no specific methods to prevent both wrinkle and breakage of microporous plastic films.

Non-patent document 3, furthermore, describes example cases where the theory proposed in patent document 1 is applied to actual production processes. Although some studies have insisted that the reduction in friction coefficient is effective for the decrease of wrinkles attributable to conveyance, but according to knowledge of the present inventors, microporous plastic films require special solution to problems with their unique friction generation mechanism, and non-patent document 3 is not successful in showing a specific method to prevent both wrinkle and breakage.

Patent document 2 proposes technique to form diamond like carbon (hereinafter abbreviated as DLC) on the surface of the conveyance roller to reduce the friction coefficient of the rubber layer at the surface or a pressurizing rubber roller that is pressed against a film roll so that the film can be wound up while eliminating air. According to knowledge of the present inventors, however, friction coefficient reduction effect of a DLC layer is developed by preventing minute deformation and reducing the real contact area of the surface by virtue of high hardness of the DLC layer, and therefore, it cannot be highly effective for the phenomenon in which the friction coefficient increases by virtue of the flexibility of microporous plastic film itself.

Compared to this, patent document 3 proposes a method in which scratches on a film is prevented by adopting a conveyance roller that has a metal surface with a decreased surface roughness and reducing its friction coefficient. According to knowledge of the present inventors, however, although it can be expected in the case of synthetic resin films with a smooth surface dealt with in patent document 3 that the friction coefficient can be reduced by decreasing protrusions on, that is, decreasing the roughness of, the roller surface and by making use of a phenomenon that is considered to be air lubrication, such air lubrication cannot be expected in the case of a microporous plastic film, which will undergo air release through micropores, and its contact with a smooth metal surface will increase the friction coefficient contrary to expectation, thus failing to prevent wrinkle and breakage.

As stated above, no techniques have been available conventionally to convey a microporous plastic film and wind it up into a roll without wrinkle or breakage.

PATENT DOCUMENTS

-   Patent document 1: JP H11-314333 A -   Patent document 2: JP 2004-251373 A -   Patent document 3: JP 2001-63884 A

NON-PATENT DOCUMENTS

-   Non-patent document 1: Hiromu Hashimoto, “Uebuhandoringu no Kiso     Riron to Oyo” (Basic Theory and Application of Web Handling, in     Japanese), 1st Ed., pub. by Converting Technical Institute Co.,     Ltd., April 2008, p.p. 131-155 -   Non-patent document 2: Hiromu Hashimoto, Convertech, July 2009     Issue, pub. by Converting Technical Institute Co., Ltd., July 2009,     p.p. 36-43 -   Non-patent document 3: Morikawa Akira, Convertech, November 2010     Issue, pub. by Converting Technical Institute Co., Ltd., November     2010, p.p. 58-63

SUMMARY OF THE INVENTION

The present invention provides a production method for microporous plastic films, which have been conventionally difficult to handle due to the existence of micropores that can cause wrinkle and breakage.

The present invention provides a microporous plastic film roll production method comprising the steps of: conveying microporous plastic film having through-holes in its interior by using a plurality of conveyance rollers at least one conveyance roller of which has a surface roughness RzJIS (μm) of 0.3≦RzJIS≦30 and has a surface made of fluorine resin, silicone rubber, or a composite material containing one of them; and winding it up into a roll.

A more preferred embodiment of the present invention provides a microporous plastic film roll production method as described above wherein the conveyance roller has a surface made of polytetrafluoroethylene as its material.

Another preferred embodiment of the present invention provides a microporous plastic film roll production method wherein the microporous plastic film has a Gurley air permeation resistance of 10 to 1,000 seconds per 100 ml.

Another preferred embodiment of the present invention provides a microporous plastic film roll production method as described above wherein the microporous plastic film has a porosity of 30% or more.

Another preferred embodiment of the present invention provides a microporous plastic film roll production method wherein the microporous plastic film has an average micropore size of 50 to 200 nm.

Another preferred embodiment of the present invention provides a microporous plastic film roll production method wherein the microporous plastic film has a cushion rate of 15% or more and less than 50%.

Another preferred embodiment of the present invention provides a microporous plastic film roll production method wherein the microporous plastic film has a thickness of 50 μm or less.

Another preferred embodiment of the present invention provides a microporous plastic film roll production method wherein the microporous plastic film has a width of 100 mm or more.

Another preferred embodiment of the present invention provides a microporous plastic film roll production method wherein the coefficient of static friction between the microporous plastic film and the conveyance roller is 0.6 or less.

Also provided is a microporous plastic film roll production method wherein the microporous plastic film is designed to be used as the separator of either a secondary battery or a capacitor.

For the present invention, the “conveyance roller” includes a means by which a microporous plastic film that is continuous in the length direction is conveyed from the upstream side to the downstream side in the production process and which is in the form of a cylinder supported in a rotatable manner.

For the present invention, the term “RzJIS” refers to the ten-point average roughness.

For the present invention, the term “fluorine resin” refers collectively to synthetic resins partly containing the fluorine element, such as ethylene based hydrocarbons.

For the present invention, the term “silicone rubber” refers to silicone resin that exhibits rubber-like elasticity, and the term “silicone resin” refers collectively to synthetic resins having a siloxane bond that contains silicon and oxygen.

For the present invention, the term “composite material” refers to a material produced by mixing a fluorine resin or a silicone resin as described above to such an extent that their properties can contribute effectively, such as one consisting of layers of rubber material or metal-plated material and a fluorine resin or silicone resin as described above added between them by coating or filling.

For the present invention, the term “microporous plastic film” refers to a thin polymer film having many minute holes in the interior of the film, in which part or all of the micropores are through-holes.

For the present invention, the term “polytetrafluoroethylene”, which may be abbreviated as PTFE, refers to a kind of fluorine resin, otherwise known as Tetrafluorideethylene. For the present invention, the term “thickness” is calculated by dividing the volume of the microporous plastic film constituting a roll by the width and length thereof, and accordingly the thickness includes the air layers that form part of the micropores.

For the present invention, the term “Gurley air permeation resistance” refers to an index of air transmittance of a film or sheet determined by the test method specified in Japanese Industrial Standard (JIS) P8117 (2009).

As the air permeability increases, air passes through the micropores in a shorter time and the Gurley air permeation resistance decreases.

For the present invention, the term “porosity” refers to the percentage portion of the cross section of a film accounted for by the micropores.

For the present invention, the term “average micropore size” refers to the average diameter of many micropores that differ in diameter.

For the present invention, the term “cushion rate” refers to the percentage change in the thickness of a sheet that takes place when a pressure is applied in the thickness direction.

Cushion rate (%)=(1−T1/T2)×100

T1: A gauge head with a diameter of 10 mm is fixed to a dial gauge supplied by Mitutoyo Corporation and a load of 50 g is applied to the gauge head to press a film specimen in the thickness direction. The thickness T1 is measured after pressing the film specimen for 30 seconds under above conditions as compared to the thickness measurement made before attaching the film specimen, which is assumed to be zero.

T2: A gauge head with a diameter of 10 mm is fixed to a dial gauge supplied by Mitutoyo Corporation and a load of 500 g is applied to the gauge head to press a film specimen in the thickness direction. The thickness T2 is measured after pressing the film specimen for 30 seconds under above conditions as compared to the thickness measurement made before attaching the film specimen, which is assumed to be zero.

For the present invention, the term “secondary battery” refers to a rechargeable battery which is also referred to as a storage battery.

For the present invention, the term “separator” refers to a functional film designed to prevent short-circuiting between electrodes. A film containing micropores and transmitting ion electrolyte can be used as a separator.

For the present invention, the term “capacitor” refers to a passive element that has an electrostatic capacity and can store and discharge electric energy.

As described below, the present invention serves to provide a microporous plastic film roll production method that can prevent wrinkle and breakage to allow a high-quality microporous plastic film to be produced with high productivity.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 gives a schematic side view according to an embodiment of the present invention.

FIG. 2 gives a schematic enlarged view of the surface of a conveyance roller according to an embodiment of the present invention.

FIG. 3 gives a separator of a secondary battery to which a microporous plastic film produced according to an embodiment of the present invention is applied.

FIG. 4 gives an enlarged plan view of a microporous plastic film produced according to an embodiment of the present invention.

FIG. 4 gives a schematic side view illustrating a method to measure the coefficient of static friction between a film and the portion of a conveyance roller that is in contact with the film.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

An example of the invention is described below with reference to drawings, taking as an example its application to a process for the production of a microporous plastic film to be used as a separator film of a secondary battery.

FIG. 1 gives a schematic side view showing the conveying and winding-up steps in a microporous plastic film roll production process according to an embodiment of the present invention.

The microporous plastic film 1 may be formed by any appropriate method. As a preferable example, it can be produced by kneading molten polyolefin resin with a highly volatile solvent in an extruder, discharging it through an die onto a cooling drum to form a gel sheet, and subjecting it to steps for stretching and orientation as required, followed by washing off the solvent and drying. In another example, it can be produced by kneading polyolefin resin with a crystal nucleating agent, discharging it through a die onto a cooling drum, and forming micropores by controlling the crystal structure without using a solvent. In another example, the microporous film 1 can be produced by combining a heat resistant polymer such as polyamide or polyimide with a solvent that differs in compatibility with it to form micropores, followed by discharging or coating. In another example, one side or both sides of a microporous polyolefin film is coated with a heat resistant material that does not impair the gas permeation function of the micropores. In still another example, a microporous film, similar to paper and nonwoven fabrics, is formed by stacking synthetic fibers.

A microporous plastic film 1 produced as described above is preferably stretched uniaxially or biaxially as required to control the pore structure and ensure high strength.

FIG. 4 gives an enlarged plan view showing an example of the microporous plastic film 1. As illustrated in the figure, any appropriate method may be used to form micropores in the plastic film 1. In a stretched and oriented resin layer, the portions around pores form fibrous columns as illustrated in the figure, which may be referred to as fibrils 18. Part or all of these micropores function as through-holes 17.

FIG. 3 gives an explanatory diagram showing a schematic exploded view of part of a tubular lithium ion secondary battery. A case 11 contains a positive electrode 14 and a negative electrode 15, with a separator 16 interposed between them to serve as an insulator to prevent short circuiting between these electrodes. The interior of the case is filled with a lithium ion electrolyte, and the separator 16 is required to serve for both insulation and transmission of ions in the electrolyte. This requirement is met effectively by a microporous plastic film 1 partly or entirely containing through-holes as produced by the production method according to the present invention.

As shown in FIG. 1, the microporous plastic film 1 is conveyed on a group of conveyance rollers 2 at a predetermined speed and wound up on a core 6 under a predetermined tension to form a film roll 12. In FIG. 1, the conveyance rollers 2 are driven by a driving source 32, such as motor, via a driving force transmission mechanism 4, such as belt or chain. The driving force transmission mechanism 4 is given a necessary tension and supported by a pulley 5. Here, it is not necessary for all conveyance rollers 2 to be driven by the driving source 32. If they are supported on bearings in a rotatable manner, they can work as idlers to assist the conveyance of the film 1. In this case, to avoid scratches and abrasion powder on the film 1, they are preferably driven indirectly via bearings, or some methods are preferably introduced to minimize the inertia of the rollers and the friction loss of the bearings.

A microporous plastic film 1 suitable for use as a battery separator and the like is generally liable to suffer deformation of micropores, leading to a hysteresis loss, increased real contact area, and then increased coefficient of static friction to objects in contact. In particular, if a plurality of conveyance rollers 2 are used to convey the film 1 as in the case shown in FIG. 1, the coefficient of static friction may increase due to the aforementioned factors in portions where rollers and the film come in contact, and in addition, air lubrication, which is commonly expected in the case of conveyance of a nonporous film, may not take place due to air escape through these micropores, leading to a large increase in the friction coefficient. As stated above, an increase in the friction coefficient will cause problems such as wrinkle and breakage on conveyance rollers or between conveyance rollers. The present invention can avoid these problems and successfully prevent the breakage of a microporous plastic film by reducing the coefficient of static friction of the surface of at least one conveyance roller of the plurality of conveyance rollers 2 to reduce the stress attributable to a difference in speed.

To reduce the coefficient of static friction of the conveyance rollers 2, the ten-point average surface roughness of the surfaces is in the range of 0.3≦RzJIS (μm)≦30. If the RzJIS is 0.3 μm or more, it will be possible to maintain a small real contact area, which tends to increase if micropores are deformed in the microporous plastic film 1, leading to a reduced coefficient of static friction. In addition, moderately rough surfaces of the conveyance rollers 2 can serve to reduce the coefficient of static friction, although the microporous plastic film 1 tends to suffer from an increase in the contact area as air escapes through micropores due to gas permeation. If the conveyance rollers 2 have too large a surface roughness, on the other hand, processing the roller surfaces will be difficult and expensive and low in accuracy if the RzJIS is more than 30 μm. If the roughness is too high as in this case, the coefficient of static friction may increase contrary to expectation as a result of an increase in the contact area between individual surface projections and the film. The RzJIS is more preferably in the range of 2≦RzJIS (μm)≦10.

As a result of intensive studies carried out by the inventors of the present invention, it has been found that reduction in the coefficient of friction of the conveyance rollers 2 to a film of a microporous plastic material that is air permeable and liable to suffer deformation of pores can be realized only by controlling the intermolecular force in the surface material of the conveyance rollers 2 instead of controlling the hardness of the contact area by, for example, using diamond-like carbon (DLC) as surface material to achieve a low friction. This is because the use of rollers 2 having a surface of a hard material can not prevent deformation of surface structures of the microporous plastic film 1, failing to decrease the real contact area.

For the present invention, therefore, a material with a small intermolecular force, such as fluorine resin, silicone rubber, or a composite material containing them, is used, instead of commonly-used rubber, as the surface material of the conveyance rollers 2 that will come in contact with the microporous plastic film 1. From the viewpoint of maintaining a high durability and ensuring uniform treatment, the thickness of a fluorine resin surface is several tens of micrometers, preferably about 10 to 100 μm. In general, fluorine resin is preferably calcined at 300 to 400° C. If it is used to coat a resin or rubber surface, it is preferable to perform molding at 100° C. or less. In this case, additional treatment by polishing will be effective to obtain a roller with a high surface accuracy. Preferred methods for forming a fluorine resin surface on the roller include coating, spraying, and fitting. Others include, for instance, covering the roller with a tape or a tube of fluorine resin. When covering the roller with a tape or a tube of fluorine resin, its thickness is preferably about several millimeters to ensure easy formation.

In the case of using silicone rubber, its thickness is preferably several millimeters, specifically about 1 to 10 mm.

The base material 2A of the conveyance roller is preferably steel, stainless steel, aluminum alloy, or CFRP.

Here, a composite material contains fluorine resin or silicone rubber as mentioned above to such an extent that their properties can contribute effectively to friction reduction. Such materials include, for instance, one consisting of layers of rubber material or metal-plated material and fluorine resin or silicone rubber added between them by coating or filling. FIG. 2 shows an example of such a composite material 9, in which a conveyance roller is composed of a base material 2A, rigid chrome-plated layer 7, and fluorine resin 8 that impregnates the rough surface. Here, in the portion in contact with the microporous plastic film 1, the material of the metal-plated layer 7 and the fluorine resin 8 are randomly distributed to allow both the advantage of friction coefficient reduction by the fluorine resin and that of wear resistance of the metal-plated layer to be realized effectively.

When such fluorine resin is added to a plated layer to form a composite material, a preferable processing method to obtain a high-strength treated surface is calcination at a high temperature as described above, and in that case, the base material 2A of the conveyance roller may be steel that is heat-treated in advance to prevent strain to take place at high temperatures.

To meet the aim of performing the aforementioned function, it may be combined with materials other than metal-plated ones, such as, for instance, ceramics, rubber, and other resins, to provide a composite material. Another effective method is to coat the roller surface with the aforementioned fluorine resin, silicone rubber, or both of them.

Here again, these composite materials contain distributed ceramics or other hard materials that work to control the wear resistance and roughness of the surface in contact with fluorine resin or silicone rubber.

The use of these materials serves to reduce the coefficient of static friction of the surface to the microporous plastic film 1.

The selection of a material having appropriate roughness and properties serves to reduce the coefficient of static friction of the surface to the microporous plastic film 1 down to such a low value that is required to prevent breakage and wrinkle. In other words, the friction coefficient can be reduced effectively only when using a material with a small intermolecular force and controlling its properties in the aforementioned range so that the contact area will be reduced effectively even if the surface structure of the microporous plastic film is deformed.

When using a composite material, in particular, it is necessary for the aforementioned roughness to be achieved not only by using a plated layer 7 but by creating a surface of fluorine resin or silicone rubber over the plated layer of the base and carrying out polishing or other final finishing steps as required so that the roughness will come in the aforementioned RzJIS range.

The coefficient of static friction is preferably 0.6 or less. A coefficient of static friction in a more preferable range of 0.5 or less can be obtained by controlling the ten-point average roughness at a high value in the aforementioned range, or by combining it with good material properties.

For example, the material of the conveyance roller 2 is more preferably polytetrafluoroethylene among other fluorine resin compounds. Various fluorine resin compounds have different features such as heat resistance and releasability depending on their components, but the aforementioned resin compound was found to be effective for reduction in friction coefficient in association with its intermolecular force. A good combination of a roughness and material properties can allow the coefficient of static friction to come in a more preferably range of 0.3 or less.

As described above, the microporous plastic film 1 is required, depending on uses, to have the ability to allow gas and liquid to permeate through the aforementioned micropores. In the case of separators for lithium ion secondary batteries as mentioned above, in particular, the electrolyte permeability is generally determined indirectly from the air permeability.

The gas permeability of a microporous plastic film can be determined from the Gurley air permeation resistance defined in JIS P8117 (2009), and a film can exhibit a suitable electrolyte permeability for separators of batteries and capacitors in the preferable range of 10 to 1,000 seconds per, 100 ml. If the Gurley air permeation resistance is 10 seconds per 100 ml or more, moderate insulating properties are maintained to permit the production of a separator that can reduce the risk of short-circuiting, and the film can have a high strength so that film breakage during conveyance can be avoided when used in combination with conveyance rollers designed for the present invention. If the Gurley air permeation resistance is 1,000 seconds per 100 ml or less, on the other hand, a large number of through-holes can be maintained to permit smooth permeation of required quantities of gas and liquid. When it is used as a separator of a lithium ion secondary battery, in particular, a high electrolyte permeability will be maintained to permit quick recharging and discharging of the batteries. The use of the aforementioned conveyance rollers 2 as part of the production method for the microporous plastic film 1 serves to produce a microporous plastic film 1 having a high functionality as separator of a battery or the like and at the same time having a reduced value of the coefficient of static friction to prevent wrinkle and breakage.

The microporous plastic film 1 is pressed against a conveyance roller under a tension T. The contact pressure in this case is calculated as follows: tension×contact angle. This contact pressure works to allow air to escape from the microporous plastic film through micropores and the deformation of micropores will cause a large friction with the conveyance roller. The microporous plastic film 1, for which the conveyance roller according to the present invention works effectively to prevent wrinkle and breakage, undergo a high degree of deformation under the contact pressure, and this degree is represented by the cushion rate as a parameter. The cushion rate indicates the percentage change in thickness of a film to which a load of 50 g or 500 g is applied in the film thickness direction via the gauge head of a dial gauge.

For cushion rate measurement, a load may be applied by any appropriate method such as the use of a spring or weight, but it is preferable to install a weight above the gauge head or the indicator so as to minimize the moment applied to the gauge head.

A microporous plastic film preferred for the present invention has a cushion rate of 15% or more and less than 50%. If the cushion rate is 15% or more, the micropores in the microporous plastic film 1 maintains a certain degree of through-hole rate, serving for smooth permeation of gas and liquid at required rates while preventing an increase in the friction with the conveyance roller. When it is used as a separator of a lithium ion secondary battery, in particular, a high electrolyte permeability will be maintained to permit quick recharging and discharging of the batteries. If the cushion rate is less than 50%, on the other hand, a moderate air permeation resistance is maintained to permit the production of a separator that can prevent short-circuiting, and film breakage during conveyance can be avoided. The use of the aforementioned conveyance rollers 2 as part of the production method for a microporous plastic film 1 having a cushion rate of 15% or more and less than 50% serves to produce a microporous plastic film 1 having a high functionality as separator of a battery or the like and at the same time having a reduced value of the coefficient of static friction, which increases with the cushion rate, to prevent wrinkle and breakage.

In addition, in order to prevent the friction coefficient from increasing considerably with an increasing cushion rate, it is preferable to control the porosity of the microporous plastic film 1 at 50% or less, particularly preferably 30% or less. If the aforementioned plastic film is used as separator of a secondary battery, the use of a microporous film with a high ion permeability and a high porosity is preferable in order to achieve a high output and shorten the recharging and discharging time. It is preferably 30% or more, more preferably about 50 to 80%. From the viewpoint of breakage prevention, a porosity of 80% or less is essential.

Here, there are some measuring methods that seem to be helpful for measuring the porosity of the microporous plastic film 1, but for the present invention, it is determined by sampling a predetermined quantity of the film 1, calculating the volume Va of the resin portion from the weight of the sample and the density of the resin that constitutes the film, calculating the volume Vb from measurements of the thickness, width, and length of the film, and substituting them into formula 1. The thickness of the film is preferably determined from continuous measurements made by applying a light emitting/receiving type or reflection type laser sensor to the conveyance roller. Other available methods include the use of a radiation or infrared ray sensor, and the use of a dial gauge under a low load to examine a specimen taken from the wound-up film 1.

$\begin{matrix} \left\lbrack {{formula}\mspace{14mu} 1} \right\rbrack & \; \\ {{{Porosity}\mspace{14mu} (\%)} = {\left\lbrack \frac{V_{b} - V_{a}}{V_{b}} \right\rbrack \times 100}} & \left( {{formula}\mspace{14mu} 1} \right) \end{matrix}$

The microporous plastic film 1 intended in an embodiment of the present invention has an average pore size of 50 to 200 nm. If a film with an average pore size is 50 nm or more is used as a separator of a lithium ion secondary battery, a certain degree of electrolyte permeability will be maintained to permit quick recharging and discharging of the batteries. If the average pore size is less than 200 nm, on the other hand, it will be possible to produce a separator that can prevent short-circuiting, and film breakage during conveyance can be avoided to some extent.

A film with a thickness of 50 μm or less, which may serve effectively as separator of a secondary battery or capacitor, will be particularly liable to suffer wrinkle and breakage if the friction coefficient is high as in the above case, and therefore, the conveyance roller 2 can be used effectively.

If the microporous plastic film 1 has a width of more than 100 mm, wrinkle will take place severely. This is partly because an alignment error occurs between conveyance rollers, leading to a moment on the microporous plastic film 1. This moment that induces wrinkle is proportional to the product of [α×width of film 1] where α represents the angle between the rotation axes of two conveyance rollers. The angle α, which represents the alignment error, is zero when the conveyance rollers are completely parallel to each other. When there exists an alignment error as represented by α, the wrinkle-inducing moment decreases as the microporous plastic film 1 decreases in width. As a result of intensive studies based on experiments, the inventors of the present invention have found that the frequency of wrinkle increases as the width decreases below 100 mm.

In addition, the microporous plastic film 1 receives strains during film production and other various processing steps, and consequently, its surface is not completely plain in most cases. Thus, in addition to the aforementioned moment, which is a geometrical factor associated with the parallelism of the rollers, there exists another factor in wrinkle that is associated with the planarity of the film. Therefore, there is a peak of wrinkle frequency that is not associated with the alignment, and in particular, wrinkle becomes more significant as the aforementioned width exceeds 500 mm. In this way, the inventors of the present invention found that although the microporous plastic film 1 becomes very difficult to handle as its width exceeds 100 mm, particularly 500 mm, the reduction in the coefficient of static friction to the conveyance rollers can serve to realize both the prevention of wrinkle and the prevention of breakage even in the case where conveyance rollers are low in parallelism or the microporous plastic film 1 is low in planarity during its conveyance.

More preferably, a method for removing wrinkles is carried out additionally after reducing the coefficient of static friction between the microporous plastic film 1 and the conveyance rollers 2 by the method described above, which will further serve to prevent wrinkle of the microporous plastic film 1 during its conveyance.

As stated previously with reference to FIG. 1, in the case where at least one roller of the plurality of conveyance rollers 2 has a surface as described above, wrinkle and breakage can be prevented effectively by reducing its coefficient of static friction. Here, all of the conveyance rollers 2 may have a coefficient of static friction as stated above, but it may be only those rollers located, for example, where breakage or wrinkle can easily take place, that have such a coefficient of static friction. At positions where slippage between a conveyance roller 2 and the film 1 should be avoided, it will be effective to use a wrinkle removing device 19 instead of reducing the friction coefficient of the conveyance roller 2. Specifically, it will be effective that all or some of conveyance rollers 2 are rollers having a friction coefficient of 0.7 or less, more preferably 0.5 or less, and removing devices are provided at positions where a decrease in the friction coefficient is not permitted, or cannot work to reduce wrinkle, such as where achieving required parallelism is difficult.

Here, the coefficient of static friction between a conveyance roller 2 and the aforementioned plastic film 1 is determined by a measuring method as described below. A method is shown in FIG. 5 where a film 1 is brought into contact at an angle of θ(rad) with a roller 2 that is fixed in a non-rotatable manner. A weight of W (N) is hung and the tension T (N) at which slippage starts is read on the spring balance scale 31, followed by calculation using formula 2. An appropriate weight W is preferably set based on the preferable tension conditions described later, and there are no specific limitations on the width of the film 1. For instance, when it is 0.1 m to ensure easy handling, the weight W is preferably from 1 N/m×0.1 m to 30 N/m×0.1 m, namely, 0.1 N to 3 N. This is because the mode of pore deformation changes leading to a variation in the values as the coefficient of static friction deviates significantly from the aforementioned load range.

In another method, the film 1 is attached to the contact head of a portable tribometer (Muse, supplied by Shinto Scientific Co., Ltd.) and the contact head is brought into contact with the roller 2, followed by making measurements. To calculate the contact pressure to the roller surface in the aforementioned tension range, the contact pressure p [Pa], tension T [N/m], and roller diameter D [m] have the following relationship: p=2T/D. For a roller diameter D of 0.1 m, for example, the contact pressure p is preferably in the range of 20 to 600 Pa when the tension is in the preferable range described above. For Muse, the weight has a mass of 0.4 N and a diameter of 0.03 m, the contact pressure p is 570 Pa, which is in the aforementioned preferable range.

$\begin{matrix} \left\lbrack {{formula}\mspace{14mu} 2} \right\rbrack & \; \\ {\mu_{s} = {\frac{1}{\theta}\ln \frac{T}{W}}} & \left( {{formula}\mspace{14mu} 2} \right) \end{matrix}$

In FIG. 1, the tension to the film 1 may be applied by the torque of the motor 31 given in FIG. 1, or particularly when conveying an easily breakable and deformable film such as microporous plastic film, it may be effective to use a dancer roller which can apply tension by a boost pressure and can control a significantly small tension. In this case, the motor 31 and the motor 32 are preferably of a speed- and rotation frequency controllable type.

An appropriately selected tension may be used to realize the effect of the invention, but from the viewpoint of breakage prevention, it is preferable to use a smaller tension as compared to cases with general resin films. For instance, it is preferably in the range of 1 N/m to 30 N/m.

The tension is more preferably in the range of 5 N/m to 20 N/m to allow breakage and wrinkle to be prevented easily while controlling the tension in the machine with appropriate accuracy.

Here, breakage caused between conveyance rollers 2 is mainly attributed to a slight difference in speed taking place between the conveyance rollers. This is because there is a non-zero speed control error when a plurality of motors are involved in driving the rollers, and in the case of the example shown in FIG. 1, a difference in speed can take place as a result of slippage between a pulley and a drive controller 4 working to drive the conveyance rollers 2 as well as an error in the outside diameters of pulleys. In these cases, the difference in speed causes stress concentration around micropores existing in the microporous plastic film, easily leading to its breakage.

The mechanism of breakage caused by a difference in speed is as follows. In the case where no slippage takes place, the strain ∈ caused in the microporous plastic film 1 pulled by the difference in speed in FIG. 1 is considered to be roughly expressed by formula 3, where V2 is the circumferential rotation speed of the conveyance roller 21 and V1 is the circumferential rotation speed of the conveyance roller 23 assuming V2>V1. According to Hooke's law, the stress σ1 caused by this strain is expressed by formula 4, where E represents Young's modulus of the microporous plastic film 1 in the length direction.

$\begin{matrix} \left\lbrack {{formula}\mspace{14mu} 3} \right\rbrack & \; \\ {ɛ \cong \frac{V_{2} - V_{1}}{V_{1}}} & \left( {{formula}\mspace{14mu} 3} \right) \\ \left\lbrack {{formula}\mspace{14mu} 4} \right\rbrack & \; \\ {\sigma_{1} = {{E\; ɛ} \cong {E\frac{V_{2} - V_{1}}{V_{2}}}}} & \left( {{formula}\mspace{14mu} 4} \right) \end{matrix}$

On the other hand, the stress σ2 caused by rotating core torque is expressed by formula 5, where T represents the tension applied during the process to a unit width of the microporous plastic film 1 and t represents the thickness of the microporous plastic film 1.

[formula 5]

σ₂ =Tt  (formula 5)

Breakage occurs when the inequality of formula 6 is met, where σb represents the rupture stress of the microporous plastic film 1. Here, σb can be determined from rupture test of the microporous plastic film 1 using a tensile tester or the like. In the case of a micropore-containing film, however, scratches tend to be caused by the blades of a continuous cutting device installed in the production process to accelerate the stress concentration in the cut portions. Consequently, breakage tends to occur at a stress that is smaller than the rupture stress obtained from the aforementioned tensile test. The inventors of the present invention found that breakage can be prevented by minimizing the value of σ1. Thus, the strain ∈ is prevented from being caused by a difference in speed and the stress is maintained below σb by decreasing the coefficient of static friction between the film and that portion of a conveyance roller 2 which comes in contact with the film. This successfully worked to prevent breakage.

[formula 6]

σ_(b)<σ₁+σ₂  (formula 6)

EXAMPLES

Given below are results of producing a microporous plastic film roll for secondary battery separators by using the microporous plastic film production method described above.

Example 1

A microporous plastic film 1 of polypropylene in which through-holes as illustrated in FIG. 4 were formed by carrying out a biaxial stretching step while controlling the crystal structure of polypropylene was conveyed by a conveyance roller 2 as illustrated in FIG. 1 and wound up continuously on a core 6 to produce a microporous plastic film roll 12. The microporous polypropylene film had a Gurley air permeation resistance of 500 seconds per 100 ml, porosity of 70%, average pore size of 100 nm, and cushion rate of 17%. The film 1 had a width of 600 mm and thickness of 60 μm. The thickness was measured with an emitting/receiving type laser sensor, and the porosity was calculated by formula 6 from the measured thickness.

Here, the gas permeability is represented by the Gurley air permeation resistance (seconds per 100 ml) as specified in MS P8117 (2001). The Gurley air permeation resistance is defined as the time period required by 100 ml of air pressured under a constant pressure to pass through a microporous film, and the Gurley air permeation resistance, that is, the air passage time, decreases as the gas permeability increases.

Here, the average pore size of the microporous plastic film 1 may be measured by any appropriate method, and can be determined by using the following measuring device under the following conditions.

Measuring device: Perm-Porometer, automatic micropore size distribution measuring device supplied by Porous Materials, Inc. Testing liquid: Fluorinert FC-40 supplied by 3M Testing temperature: 25° C.

Testing gas: air

Analyzing software: Capwin Measuring conditions: automatic measurement under default conditions: Capillary Flow Porometry—Wet up, Dry down Conversion equation: d=Cγ/P×10̂3 d: pore diameter (nm), C: constant, γ: Fluorinert surface tension (16 mN/m), P: pressure (Pa)

As illustrated in FIG. 1, the conveyance roller 21, conveyance roller 23, and conveyance roller 24 driven by a motor 32 via a belt, and controlled at a constant speed immediately before starting winding-up. Here, a load measuring device is provided on the bearing of the conveyance roller 22 to measure the tension. The conveyance roller 22 is driven by the film 1 instead of the motor 32, preventing the direction of the resultant tensional force from being varied by the frictional force on the roller.

The core 6 is supported rotatably on the winding-up shaft and driven by the motor 31 in such a manner that a constant tension is maintained. For this example, a composite material of metal and a tetrafluoroethylene—perfluoroalkylvinyl ether copolymer (PFA), which is a fluorine resin, was formed on the surfaces of the conveyance roller 21, conveyance roller 22, conveyance roller 23, and conveyance roller 24 as illustrated in FIG. 2 so that the portions of the four rollers, that is, conveyance rollers 21 to 24, that come in contact with the film 1 have a coefficient of static friction of 0.7 or less. To represent the surface roughness of the roller surfaces, their ten-point average roughness RzJIS was measured with a contact type surface roughness measuring device supplied by Mitutoyo Corporation using a diamond stylus with a stylus end radius of 2 μm at a measuring force of 0.75 mN as specified in Japanese Industrial Standards JIS B0601 (2001).

Consequently, the coefficient of static friction to the film 1 measured with Muse supplied by Shinto Scientific Co., Ltd. was 0.55. Here, the contact pressure p applied by the contact head was about 570 Pa.

The production conditions for the microporous plastic film roll 1 included a conveyance speed of 10 m/min and tension of 20 N/m, and a film roll was ejected by an automatic roll changer each time the winding-up of a 1,000 m length is completed.

The combinations of these conditions are summarized in Table 1.

Example 2

A film 1 with a thickness of 20 μm was wound up under the same conditions as in Example 1 to provide a microporous plastic film roll 12. The film had subsequently the same porosity as in the example, but its Gurley air permeation resistance was 100 seconds per 100 ml as a result of a smaller thickness.

Example 3

The same procedure as in Example 2 was carried out except that a composite film of metal and polytetrafluoroethylene (PTFE) was formed on the surfaces of the conveyance rollers 21 to 24 as illustrated in FIG. 2 so that the portions of the four rollers coming in contact with the film 1 had a coefficient of static friction of 0.5 or less. To represent the surface roughness of the roller surfaces, their ten-point average roughness was measured under the same conditions as in Example 1.

Example 4

A microporous plastic film roll 12 was produced by the same procedure as in Example 3 except that a film 1 with a Gurley air permeation resistance of 400 seconds per 100 ml and porosity of 40% was wound up.

Example 5

A microporous plastic film roll 12 was produced by the same procedure as in Example 3 except that the four rollers used, that is, the conveyance rollers 21 to 24, had surfaces with a small roughness such that their portions that come in contact with the film 1 had a coefficient of friction of 0.5 or less.

Example 6

A microporous plastic film roll 12 was produced by the same procedure as in Example 3 except that a film 1 with a Gurley air permeation resistance of 900 seconds per 100 ml and porosity of 30% was wound up.

Comparative Example 1

A film 1 with a Gurley air permeation resistance of 100 seconds per 100 ml, porosity of 70%, and thickness of 20 μm as in Examples 2 and 3 was conveyed by four rollers, that is, the conveyance rollers 21 to 24, in which the portions coming in contact with the film 1 had surfaces plated with hard chromium (Hcr) and having a surface roughness RzJIS of 0.1 μm, and it was wound up to produce a microporous plastic film 12.

Comparative Example 2

A film 1 with a Gurley air permeation resistance of 100 seconds per 100 ml, porosity of 70%, and thickness of 20 μm as in Examples 2 and 3 was conveyed by four rollers, that is, the conveyance rollers 21 to 24, in which the portions coming in contact with the film 1 had surfaces coated with diamond like carbon (DLC) and having a surface roughness RzJIS of 3 μm, and it was wound up to produce a microporous plastic film 12.

Comparative Example 3

A film 1 with a porosity of 0% that contained no effective through-holes required for the film to serve as a separator for secondary batteries was conveyed by the same conveyance rollers as used in Example 3 and wound up to produce a biaxially stretched polypropylene film roll.

Comparative Example 4

A film 1 with a Gurley air permeation resistance of 100 seconds per 100 ml, porosity of 70%, and thickness of 20 μm as in Example 2 was conveyed by four rollers, that is, the conveyance rollers 21 to 24, in which the portions coming in contact with the film 1 had surfaces formed of a composite film of TFE and metal and having a surface roughness RzJIS of 0.1 μm, and it was wound up to produce a microporous plastic film 12.

Table 1 shows results of the production of the microporous plastic film rolls 1 for secondary separators conducted in Examples and Comparative examples.

Here, the criteria for wrinkle were as follows: “C” for cases where wrinkles formed during conveyance left in the film roll 1 and wrinkles were found in the wound-up roll, “B” for cases where wrinkles were found in the film being conveyed, but not found in the wound-up roll, and “A” for the other cases.

Here, the criteria for breakage were as follows: “C” for cases where breakage took place before the completion of winding-up of a length of 1,000 m, “B” for cases where breakage took place once or more times before the completion of winding-up of a length of 90,000 m, and “A” for the other cases.

The performance of a separator for secondary battery was judged according to the Gurley air permeation resistance. A preferred separator for secondary battery allows ions to pass without resistance through minute through-holes that do not cause dielectric breakdown. For its performance, a higher Gurley air permeation resistance is more preferable. The criteria were as follows: “C” for cases where the Gurley air permeation resistance was 1,000 seconds per 100 ml or more, “B” for cases where it was 200 to 1,000 seconds per 100 ml, and “A” for cases where it was 10 to 200 seconds per 100 ml or less.

In Example 1, as seen from Table 1, the portion of a conveyance roller 2 coming in contact with the microporous plastic film 1 was formed a PFA composite material so that the coefficient of friction to the film 1 was 0.6 or less, and consequently, a microporous plastic film roll was able to be produced in a state where wrinkle was completely prevented and the breakage frequency was very low while realizing a porosity and Gurley air permeation resistance required for a separator for secondary battery.

In Example 2, a decrease in thickness led to an improved gas permeability and an increased risk of breakage and wrinkle, but wrinkle and breakage frequency were minimized by using a PFA composite film to ensure a coefficient of static friction of 0.6 or less as in Example 1.

In Example 3, a film that was thin, low in porosity, and difficult to handle as in Example 2 was used and the PTFE composite film realized a coefficient of static friction of 0.6 or less, serving to prevent wrinkle and breakage.

In Example 4, a decrease in porosity led to a slightly inferior gas permeability, but the use of the same PTFE composite film as in Example 3 served to realized a further decrease in the friction coefficient, leading to good results in wrinkle and breakage evaluations as in the other examples.

In Example 5, the composite film had a smaller roughness than in Examples 1 to 4 and accordingly had a slightly higher coefficient of static friction to cause wrinkle, but the use of PTFE served to realize a coefficient of static friction of 0.6 or less, leading to a good wound-up film free of wrinkles.

In Example 6, the film had a decreased gas permeability (increased Gurley air permeation resistance) and also had a low cushion rate, and accordingly, it had the lowest coefficient of static friction, resulting in a conveyance performance as high as that of common films when a composite film equivalent to that used in Example 4 was adopted.

As compared to these, the conveyance roller prepared in Comparative example 1 had a Hcr-plated surface with a small roughness, and had a large contact area and large friction, depending on the gas permeability and cushion rate of the microporous plastic film, leading to a coefficient of static friction largely exceeding 0.6. Consequently, the wound-up film roll was found to contain wrinkled that would cause deterioration in separator performance and breakage took place at a high frequency, resulting in a low productivity.

In Comparative example 2, although the coefficient of friction was improved from Comparative example 1 by coating the surfaces of the conveyance rollers with DLC, the coefficient of static friction was still more than 0.6 and failed to avoid breakage by absorbing the difference in speed between the conveyance rollers.

In Comparative example 3, the transparent polypropylene film did not contain effective micropores required for a separator, and although conveyance was performed without any problems attributable to micropores, results showed that the film did not have gas permeability required for a buttery separator.

In Comparative example 4, the surfaces of the conveyance rollers, which were formed of a composite material of PTFE, was too low in roughness to achieve a reduce in the friction coefficient down to a required level, and failed to avoid breakage by absorbing the difference in speed between the conveyance rollers.

In this way, the present invention serves to produce a microporous plastic film roll with a preferred gas permeability for a separator of a secondary battery, by conveying and winding up the film without wrinkle and breakage.

TABLE 1 Gurley air Roughness Coefficient permeation Average Evaluation results Surface RzJIS of static resistance Porosity Cushion rate pore size Thickness air material (μm) friction (sec/100 ml) (%) (%) (nm) (μm) wrinkle breakage permeability Example 1 PFA 3 0.55 500 70 17 100 60 A B B Example 2 PFA 3 0.55 100 70 22 100 20 B B A Example 3 PTFE 3 0.35 100 70 22 100 20 A A A Example 4 PTFE 3 0.30 400 40 18  60 20 A A B Example 5 PTFE 0.7 0.45 100 70 22 100 20 B A A Example 6 PTFE 3 0.20 900 30 15  50 20 A A B Comparative Hcr 0.1 1.20 100 70 22 100 20 C C A example 1 Comparative DLC 3 0.80 100 70 22 100 20 B C A example 2 Comparative PTFE 3 0.20 — 0 0 — 20 A A C example 3 Comparative PTFE 0.1 0.80 100 70 22 100 20 B C A example 4

The present invention can be applied not only to separators of secondary batteries but also to other various applications where microporous plastic films can be adopted including, but not limited to, separators of capacitors, separation films, membrane filters, base material of optical reflection coatings, and printable films.

EXPLANATION OF NUMERALS

-   1 microporous plastic film -   12 microporous plastic film roll -   2 conveyance rollers -   21 conveyance roller A -   22 conveyance roller B -   23 conveyance roller C -   2A base of conveyance roller -   3 driving source -   4 driving force transmission mechanism -   5 pulley -   6 core -   7 metal-plated layer -   8 fluorine resin layer -   9 composite material -   10 schematic exploded view of a lithium ion secondary battery -   11 case -   13 electrode tab -   14 positive electrode -   15 negative electrode -   16 separator made of microporous plastic film -   17 through-hole -   18 fibrils -   19 wrinkle remover -   30 weight -   31 spring scale -   A conveyance direction -   W mass of weight 

1. A microporous plastic film roll production method comprising the steps of: conveying microporous plastic film having through-holes in its interior by using a plurality of conveyance rollers at least one conveyance roller of which has a surface roughness RzJIS (μm) of 0.3≦RzJIS≦30 and has a surface made of fluorine resin, silicone rubber, or a composite material containing one of them; and winding it up in a roll.
 2. A microporous plastic film roll production method as described in claim 1 wherein the surface of the conveyance roller is made of polytetrafluoroethylene.
 3. A microporous plastic film roll production method as described in claim 1 wherein the microporous plastic film has a Gurley air permeation resistance of 10 to 1,000 seconds per 100 ml.
 4. A microporous plastic film roll production method as described in claim 1 wherein the microporous plastic film has a porosity of 30% or more.
 5. A microporous plastic film roll production method as described in claim 1 wherein the microporous plastic film has an average micropore size of 50 to 200 nm.
 6. A microporous plastic film roll production method as described in claim 1 wherein the microporous plastic film has a cushion rate of 15% or more and less than 50%.
 7. A microporous plastic film roll production method as described in claim 1 wherein the microporous plastic film has a thickness of 50 μm or less.
 8. A plastic film roll production method as described in claim 1 wherein the microporous plastic film has a width of 100 mm or more.
 9. A microporous plastic film roll production method as described in claim 1 to wherein the coefficient of static friction between the microporous plastic film and the conveyance roller is 0.6 or less.
 10. A microporous plastic film roll production method as described in claim 1 wherein the microporous plastic film is designed to serve as a separator of secondary batteries or capacitors. 